Amanda Knox smoked marijuana and had sex with her boyfriend on the night that her British flatmate was murdered in the house they shared in the Italian hill town of Perugia, a court has heard.

Giving evidence for the first time in her murder trial, Miss Knox proclaimed her innocence and said she was nowhere near the hillside cottage where Meredith Kercher, 21, was brutally murdered on the night of Nov 1, 2007.

Instead she spent the evening with her Italian boyfriend, Raffaele Sollecito, although he has said he cannot remember whether she was with him or not.

"On Nov 1, I told Raffaele that I wanted to watch a movie so we went to his place," Miss Knox, 21, told the court in Perugia.

After dinner, they went upstairs to his bedroom.

"I sat on the bed, he sat at his desk, he prepared the joint and then we smoked it together," she said.

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The prosecution alleges, however, that in a sex game which turned violent, Mr Sollecito held down the British student while a third defendant, Rudy Guede, sexually assaulted her and Miss Knox stabbed her three times in the neck.

Guede, 22, an Ivory Coast-born drifter and casual labourer, was found guilty of being involved in the murder in a separate trial last year and sentenced to 30 years in prison.

Miss Knox also recounted the last time she saw the Leeds University student, hours before the murder.

They talked about the Halloween party they had gone to the night before and Miss Knox noticed that the young British woman still had traces of makeup on her face, having dressed up as a vampire.

Miss Knox then had something to eat with her boyfriend while Miss Kercher stayed in her bedroom.

"She left her room, said 'bye,' walked out the door," Miss Knox said in Italian.

"That was the last time I saw her."

The next morning, the American and her boyfriend strolled over to the cottage and quickly noticed that something was amiss. Miss Knox thought it was "strange" that the front door was open.

"I called 'Is anyone there?' No one answered. I went to my room and changed, went to the bathroom and saw spots of blood there.

"I had a shower and on the way back to my room I saw blood on floor. I thought: 'Hmm, strange.'

"I put on my clothes in my room, then I went to the other bathroom to brush my hair. I saw traces of faeces in the toilet," which she said she found disgusting.

"I called Meredith, who didn't answer."

She then called one of her two Italian flatmates, Filomena Romanelli, who returned to the house and found her window had been broken in an apparent burglary.

"We found Meredith's door was locked," said Miss Knox. "Filomena was saying 'Mamma mia, it's never locked!'. I said that sometimes Meredith locked it when she had a shower or when she went to England."

A male friend broke down the door and found Miss Kercher's half-naked body lying on the floor in a pool of her own blood.

"I heard there was a body in there. There was lots of confusion," she said. "I was in shock. I couldn't believe what had happened, couldn't accept it."

Miss Knox appeared confident and composed in court, although her voice sometimes shook. She had a cold sore on her upper lip which her family said was stress-related.

She repeated accusations that police hit her on the back of the head twice and placed her under enormous pressure to blame a Congolese barman, Patrick Lumumba, for the murder.

Mr Lumumba was briefly jailed but then found to have had nothing to do with the crime and is seeking defamation damages from Miss Knox.

There will be a two month break in the trial over the summer and it is expected to last at least until the autumn.